The present invention relates to an all wheel drive system for a motor vehicle and more specifically, to an all wheel drive system including a pair of front halfshaft assemblies, a power take-off unit, a first propshaft assembly, a second propshaft assembly, a means for controlling torque transmission from the front halfshaft assemblies to the rear halfshaft assemblies, a rear differential, a pair of rear halfshaft assemblies and a torque arm mounting system.
There are generally four (4) main types of automotive driveline systems. More specifically, there exists a full-time front wheel drive system, a full-time rear wheel drive system, a part-time four wheel drive system, and an all wheel drive system. Most commonly, the systems are distinguished by the delivery of power to different combinations of drive wheels, i.e., front drive wheels, rear drive wheels or some combination thereof. In addition to delivering power to a particular combination of drive wheels, most drive systems permit the respectively driven wheels to rotate at different speeds. For example, the outside wheels must rotate faster than the inside drive wheels, and the front wheels must normally rotate faster than the rear drive wheels while driving in a corner.
Often, the inside and outside drive wheels of a two wheel drive system are connected to a differential mechanism which permits the opposite drive wheels to rotate at different speeds while delivering an approximately equal amounts of drive torque to each. Delivering approximately equal amount of torque to the drive wheels independently of the rotational speed sometimes significantly limits the total amount of torque which can be delivered to the drive wheels when one of the drive wheels loses traction. For example, if either drive wheel does not have sufficient traction to sustain its portion of the drive torque, the amount of drive torque which can be delivered to the other drive wheel is similarly reduced.
In an attempt to overcome this problem, there are certain modifications to differential performance which support unequal distributions of torque between the drive wheels. The unequal distributions of torque are supported by resisting any differential rotation between drive wheels. A limited slip differential modifies a conventional differential by including a frictional clutch mechanism which resists any relative rotation between the drive wheels. Unequal torque distribution between drive wheels is supported by sacrificing some of the differential capacity to support unequal rotational speeds between the drive wheels during cornering.
Recently, all wheel drive vehicles have been gaining popularity as a way to enhance traction capability. Instead of dividing drive power between only two wheels of a vehicle either in the front or the rear, all wheel drive vehicles divide power between all four wheels. As a result, each wheel is required to support a smaller portion of the total drive torque. However, in addition to delivering power to both the front and rear drive axle, all wheel drives must also permit the two axles to rotate at different speeds. Accordingly, driveshafts to the front and rear axles are often interconnected by a differential mechanism which permits the front and rear drive axles to rotate at different speeds while delivering approximately equal amounts of torque. Part-time four wheel drive systems permit a vehicle operator to selectively connect a second drive axle to the vehicle driveline when adverse traction conditions are encountered. It should be noted however, that if both front and rear axles are permanently interconnected by a differential mechanism, more power is expended by the drivetrain delivering power to two drive axles in comparison to delivering power to only one of the axles. Thus, adequate traction is available for a single pair of drive wheels to support the delivery of drive power and the further division of drive power among more than two wheels is not necessary. Significant power losses and reduced gas mileage sometimes occur as a result of the unnecessary transmission of drive power to additional wheels.
Part-time four wheel drive systems rely on operator judgment to select between two and four wheel drive modes. Driveshafts to the front and rear drive axles are generally coupled together in the four wheel drive mode, thereby preventing the two axles from rotating at different speeds. The drive wheels of one or the other axles tend to skid in response to courses of travel which require, the front and rear axles to rotate at different speeds. Accordingly, significant power losses occur in the four wheel drive mode from the tendency of one of the axles to brake the vehicle. Power is delivered to either the front or rear axle depending on whichever axle is required to rotate slower to maintain traction. This makes for unpredictable changes in vehicle handling characteristics by switching between effective front or rear wheel drive. Further, part-time four wheel drive vehicles experience the same loss of traction as two wheel drive vehicles until the four wheel drive mode is engaged.
It is also known in the art to provide a conventional differential interconnecting front and rear drive axles with a limited slip differential to further enhance traction capabilities of all wheel drive vehicles. The limited slip differential supports unequal distributions of torque between drive axles, but it also resist relative rotation between the axles. Accordingly, the same power losses occur from permanently driving an additional axle, and drive torque is unpredictably divided between the front and rear axles in response to situations requiring the drive axles to rotate at different speeds.
U.S. Pat. No. 5,782,328 (xe2x80x9cthe 328 patentxe2x80x9d), to Warn Industries, Inc. describes the use of a bi-directional clutch in combination with a transfer case. More specifically, the ""328 patent describes a transfer case for a vehicle having two output shafts, a gear reduction assembly, a coupling mechanism and an overrunning roller clutch for selectively producing driving of one shaft only or both shafts concurrently. The coupling mechanism selectively couples one output shaft to either (1) an input shaft, (2) the gear reduction assembly, or (3) a neutral position. The overrunning clutch has an inner race, and outer race, and rollers located between the races. Drag shoes are positioned to frictionally slide on a drag surface of a selectively grounded member to retard the rollers. A resilient band urges the drag shoes against the drag surface. When the drag shoes rotate at a sufficient speed they disengage from the drag surface so as to provide no force to retard the rollers. When the ground member is grounded it provides the drag surface for the drag shoes. When the ground member is ungrounded it is free to rotate and the drag shoes do not provide a drag force to bias the rollers. A latch may be coupled to the inner race to engage a roller cage to prevent high speed lock-up. A drag ring is located inside the outer race and provides a drag force on the rollers to advance the rollers when it is desired, e.g., when front wheel compression braking is advantageous. An actuator assembly is provided with a variable speed drive for shifting the transfer case between modes of operation.
U.S. Pat. No. 5,195,604, also to Warn Industries, Inc., describes a bi-directional clutch for use with a braking system. More specifically, the ""604 patent describes a clutch mechanism for a drive train of a four-wheel drive vehicle for controlling the application of braking power between the front and rear wheels as applied through the drive train. A driving shaft is connected to a driven shaft through a roller clutch assembly and the driven shaft is coupled to a wheel set equipped with anti-lock brakes. The driving shaft has a drive cam race and a driven shaft has a cylindrical race. A cage carries the clutch rollers and is provided with drag shoes biased against a fixed drag ring. The rollers captured by the cage continuously drag against the rotative urging of the driving shaft whereby the rollers engage the cylindrical race with the driving shaft inducing driving rotation of the driven shaft, and disengage with the driving shaft inducing braking rotation of the driven shaft.
U.S. Pat. No. 4,605,087 to Volkswagenwerk Aktiengesellschaft discloses an all wheel drive system wherein the wheels of only one axle are driven continuously, while the drive for the other wheels of the second axle can be obtained automatically by means of a viscosity clutch in the driving train between the front axle and the rear axle so that the front and rear axles are coupled with each other, essentially rigidly according to the torque. U.S. Pat. No. 4,605,087 discloses a device constituting an overrunning device which is automatically coupled and uncoupled according to the direction of the torque, e.g., such as a grip roller overrunning device. According to the invention, because of the presence of the overrunning device in the otherwise all wheel drive system, the possibility of locking of the front wheels could cause locking of the rear wheels and through the driving train is eliminated because the overrunning device automatically interrupts transmission of the torque in this direction through the drivetrain.
It is an object of the present invention to provide an all wheel drive system which improves vehicle traction and handling during adverse driving conditions.
It is an object of the present invention to provide an all wheel drive system which incorporates a speed sensing torque transfer system which senses speed difference between the front differential and rear differential and transfers torque upon such speed difference.
It is an object of the present invention to provide an all wheel drive system which senses speed difference between the front wheels and rear wheels and transfer torque upon such speed difference.
It is a further object of the present invention to provide an all wheel drive system which is under normal operating conditions an all wheel drive system (AWD) which operates with generally 100% torque to the front wheel drive system and if the front wheels begin to slip the front differential rotates at a higher speed than the rear differential consequently with the power take-off unit and the front propshaft causing the rear propshaft to rotate at a higher speed such that the speed sensing torque transfer device senses the speed difference between the inputs of the rear propshaft and the rear differential and engages whereby the speed sensing torque transfer device transmits torque to the rear differential and wheels until the slip condition at the front wheels is resolved and all wheels are thereby rotating at the same speed.
It is a further object of the present invention to provide an unsymmetrical torque arm for use in conjunction with the speed sensing torque transfer system and rear differential to support the rear differential.
It is yet a further object of the present invention to provide a torque arm module including a torque arm which is capable of shielding the speed sensing torque transfer system and rear differential rear propshaft from excessive heat radiated by the exhaust system of the motor vehicle.
It is yet a further object of the present invention to provide a speed sensing torque transfer system which allows for overrunning of the rear axle pinion relative to the front axle pinion without transfer of torque in both forward and reverse directions.
It is yet another object of the present invention to provide a speed sensing torque transfer system which allows for tuning of the driveline system to accommodate torque transfer to the rear wheels as required during conditions of front wheel slip without transmission of nominal driving torques when driving on high xcexc surfaces where AWD is not required.
It is another object of the present invention to provide an all wheel drive system including a speed sensing torque transfer system which compensates for tire size variation, tire wear and spare tire or spare wheel usage.
It is yet still a further object of the present invention to provide an all wheel drive system including a viscous coupling, bi-directional clutch, and a rear differential having a gear ratio greater than the power take-off gear ratio.
It is yet a further object of the present invention to provide an all wheel drive system including a viscous coupling and a bi-directional clutch whereby the viscous coupling also acts to dampen the engagement of the bi-directional clutch by reducing the amount of instantaneous transfer of torque.
It is yet another object of the present invention to provide an all wheel drive system including a torque arm mount with an asymmetric configuration which is attached to the front face of the rear differential housing and the motor vehicle undercarriage to reduce the torque reaction load of the rear differential.
It is still a further object of the present invention to provide an all wheel drive system having an asymmetrical torque arm layout which provides improved packaging flexibility and reduced packaging requirements and further acts as a heat shield between the exhaust system and the temperature sensitive driveline components.
It is yet still a further object of the present invention to provide an all wheel drive system having a bi-directional clutch connected operationally to a viscous coupling and rear differential with a gear ratio that is greater than the gear ratio of the front differential, allowing the rear wheels to overrun the front wheels.